This application claims the benefit under 35 U.S.C. xc2xa7 365 of International Application PCT/EP99/07039, filed Sep. 22, 1999, which was published in accordance with PCT Article 21(2) on Mar. 30, 2000 in English, and which claims the benefit of EP 98402335/8, filed Sep. 23, 1998.
The invention relates to the field of disturbance detection in a data signal.
Disturbance detection in a data signal is used to detect anomalies in behaviour of the data signal. The data signal may for example be a readout signal in an optical drive like a CD or DVDxe2x80x94player or any other type of data reading device. Disturbance detection is required to initiate a freeze of some parts of a data acquisition and servo processing system while data is not usable due to for example black dots, scratches, silver dots or fingerprints on a data carrier. It is advantageous to recognise disturbances as fast as possible in order to fix the data acquisition and to release it again afterwards.
The data signal has a dynamic behaviour, which may be confined between an upper and a lower envelope. In case of a disturbance in the data signal the dynamic behaviour is modified and a difference value which is equal to the subtraction of the values of the upper and lower envelope between each other may become smaller than a predetermined border value. Monitoring the difference value allows determining whether it is smaller than the predetermined border value in which case a disturbance in the data signal is detected.
However, a delay occurs between the occurrence of a defect and its detection because the envelope signal in analogue circuits is formed from the data signal by a peak detection circuit or a low pas filter. In digital devices, the upper envelope is set to a value equal to a value of the data signal if the value of the latter is greater than the upper envelope value. If during a determined period of time the upper envelope value has not been increased then it will be decreased stepwise by a predetermined value.
The lower envelope is, in a way similar to the upper envelope, set to a value equal to a value of the data signal if the value of the latter is smaller than the lower envelope value. If during a determined period of time the lower envelope value has not been decreased then it will be increased by the predetermined value.
Hence if the data signal is disturbed, e.g. it adopts a relatively low dynamic behaviour, the upper and/or lower envelope values tend to approach the data signal value and the difference value becomes smaller than the predetermined border value. A time delay in which the upper or lower envelope values reach a value near to the data signal value depends on the period of time, as the upper or lower envelope values in case of fast changes follow the data signal value, especially, if the envelope values stepwise decrease or increase as mentioned above. The time lapse appears to be relatively long in most cases of disturbance. As a consequence the detection of the disturbance, which is done after the time lapse, is delayed by a relatively long time following the effective occurrence of the disturbance.
It is an object of the present invention to reduce the time lapse occurring between a disturbance in the data signal and the detection of the disturbance.
It is another object of the present invention to have a time lapse of a determined length between the disturbance and the detection of the disturbance.
The problem has been solved according to the features in the independent claims. Advantageous embodiments are described in dependent claims.
The disadvantage of a delay between data and envelope signal will be avoided if instead of a usual envelope signal an envelope signal is used which corresponds more to an evaluation signal, which is formed in such a manner that values of the envelope signal are set equal to a corresponding value of the data signal taken in a predetermined time interval. Setting values of an envelope signal equal to a corresponding value of the data signal taken in a predetermined time interval has the advantages that an envelope signal has been generated having a constant value for a predetermined time period and also follows very quickly the value of the current data signal independent whether the data signal increases or decreases. Due to setting the values of an envelope signal equal to a corresponding value of the data signal taken in a predetermined time interval a delay between the data signal and the new kind of envelope signal corresponds as maximum to the length of the predetermined time interval and it is independent from changes in the data signal. The new kind of envelope signal follows immediately the data signal independent of an increasing or decreasing data signal. Therefore, a disturbance signal very fast can be generated by evaluating the new kind of envelope signal. Such disturbance signals are for example the well known defect or mirror signal.
The predetermined time interval is equal or longer as a maximum run length of the data signal. The maximum run length of the data signal corresponds for example to 11 clock periods in case of a CD. The upper or lower or both envelope signals have to be formed in the manner as mentioned above dependent on the manner in which the data signal has been generated and according to the kind of disturbance that has to be detected in a fast manner. For example, a threshold value for the upper envelope curve can be used to generate a defect signal in case of a direct current coupled data signal as it is well known from the prior art. The mirror signal is similar generated in such a case. The difference between the upper and lower envelope curve is mostly used in case of an alternating current coupled data signal. The invention is applicable to any kind of data signal and disturbance signal generation.
The values of an upper or lower envelope signal are set equal to respectively a maximum and a minimum value of the data signal taken before in a predetermined time interval and the computing of the difference value between an upper and a lower envelope signal is done in a time interval subsequent to said predetermined time interval in which the envelope signals have been formed. In a preferred embodiment the predetermined time interval and subsequent time intervals are taken to be of equal length.
Several predetermined time intervals are used in an overlapped manner in a further embodiment for a further reduced access time in which the envelope value will be set to a corresponding value of the data signal. In such a way different periods can be generated for example dependent on the direction of the data signal. The predetermined time intervals are therefore arranged in a phase-shifted manner.
Values of a lower envelope signal are preferred set equal to respectively a first average value and a second average value of the data signal taken before in predetermined time intervals of different length to generate a mirror signal.
According to an embodiment of the invention, a method for detecting a disturbance in a data signal comprises the steps of
obtaining a lower and upper envelope signal for the data signal by setting the values of the lower and the upper envelope signal equal to respectively a minimum and a maximum value of the data signal taken in a predetermined time interval.
computing a difference value by substracting from each other values of the lower from the upper envelope signals,
determining a disturbance if the difference value is smaller than a predetermined border value.
A device for detecting a disturbance in a data signal comprises
an envelope generator, an input of which receives the data signal and providing an envelope value according to a value of the data signal received in a predetermined time interval,
a disturbance signal generator which receives signals representative of an envelope generated by the envelope generator and
a register interface which outputs to the envelope generator a signal representative of a predetermined time interval and to the defect signal generator a signal representative of a predetermined evaluation value.
For carrying out the method as mentioned above said disturbance signal generator receives signals representative of an upper and a lower envelope generated by the envelope generator giving to the upper and lower envelope value respectively a maximum and a minimum value of the data signal received in the predetermined time interval. The register interface provides a predetermined border value as evaluation value and the disturbance signal generator is in this embodiment a defect signal generator which computes a difference value by subtracting the lower envelope from the upper envelope. The defect signal generator generates a defect signal if the difference value is smaller than the predetermined border value.